How do I straighten out my loop cuts?
What I'm understanding is that you'd like to bring the vertices of the back face of your object to be directly behind the corresponding vertices of the front face, down global Y (since you are in Front view)

If the back faces are the same shape as your front faces, and shifted, (not rotated) with respect to the front faces, you can set your snap to Active and Vertex, select all the vertices you want to move, making a convenient vertex the last selected (and therefore active) and GShift
Y the vertices, snapping the active vertex to its counterpart in XZ.

If not, another possible move - 



Select the back vertices, and (without Automatic Merge switched on)   G G slide them down their edges until they meet their front partners. (You may have to do this in more than one bite). Then  GY them to your required depth.

Or.. it may be more economical to delete the back faces, and make a brand new  extrusion in Y (EY).

It's possible that the result is not the one you're looking for..